Film stack having under layer for preventing pinhole defects

ABSTRACT

A film stack is provided in which a first film including a first polymer directly contacts a surface of a substrate at which a given material is exposed. A second film, which can include a second polymer other than the first polymer, is formed to have an inner surface contacting the first film. The second film can have a thickness at which a free energy of the second film would be negative if the second film were disposed directly on the substrate. Desirably, the resulting second film is substantially free of dewetting defects.

CROSS-REFERENCE TO RELATED APPLICATION

The present application is a continuation of U.S. application Ser. No.10/880,818 filed Jun. 30, 2004 now U.S. Pat. No. 7,267,863, thedisclosure of which is incorporated herein by reference.

BACKGROUND OF THE INVENTION

The present invention relates to the processing of thin films, such asthose used in the processing of very small structures includingmicroelectronic devices.

As the size of microelectronic devices is reduced from one generation tothe next, the demands placed on photolithography become ever greater.One consequence of the increased demands is the need to reduce thethickness of polymer films used as anti-reflective coatings (ARCs) andphotoresist imaging layers. The demands are especially severe forcertain types of applications such as first minima ARCs, thin imaginglayers for bi-layer and tri-layer lithography schemes, and thin imaginglayers used in photomask production.

One of the major problems associated with the use of thin polymer filmsfor these applications has been the appearance of dewetting defectsincluding “pinhole” defects. Dewetting defects usually occur due tolong-range van der Waals forces. Due to Van der Waals forces, localizedthinning of a polymer film on a substrate occurs when the polymer filmhas insufficient thickness to overcome a tendency to dewet from thesubstrate. An example of this phenomenon is illustrated in FIG. 1 for abottom anti-reflective coating (BARC) layer disposed on a substrate ofsilicon dioxide. FIG. 1 illustrates a free energy curve 10 for a BARClayer disposed on a substrate of silicon dioxide, and a second curve 12being the second derivative of the free energy curve 10. The BARC layerbecomes unstable and has a tendency to dewet catastrophically at athickness (50 nm) below which the free energy curve 10 turns sharplylower and heads negative. Such catastrophic dewetting is referred to asspinodal dewetting. The location of the zero in the second curve 12illustrating the second derivative of free energy indicates a crossoverpoint at about 85 nm between a film that dewets spinodally below thatthickness and dewets via nucleation and growth of holes above thatthickness. As further shown in FIG. 1, as the overall film thickness isincreased, the free energy of the film passes through a maximum andstarts to decrease slowly as the film thickness continues to beincreased. In the thickness regime just beyond the thickness at whichthe film spinodally dewets, the film is metastable can dewet vianucleation and growth of holes. If the film thickness at some localizedpoint in the film falls below the 85 nm thickness of the crossoverpoint, the film becomes locally unstable and dewets spinodally. A BARCfilm having a thickness of less than or equal to 80 nm, which is lessthan the crossover point thickness of 85 nm, is highly unstable, anddewets spinodally, rapidly dewetting to droplets. On the other hand, aBARC film having a thickness of 110 nm, does not dewet spinodally, butdewets locally via nucleation and growth of holes. However, a BARC filmhaving a thickness of 200 nm, is so far from the crossover point on thefree energy diagram that random local fluctuations in film thickness nolonger result in local instability of the film.

Heretofore, there has been no known solution to this problem other thanto increase the thickness of the film that is prone to dewet.Unfortunately, increase the thickness is not permitted, because advancedlithography processes call for reductions rather than increases in filmthicknesses, for the following reasons. A thickened BARC filmunnecessarily increases the difficulty of etching through the BARC film.A thickened photoresist imaging layer also increases risk of linepattern collapse, as well as reduces the process window.

Currently, it is common to utilize surface treatments such ashexamethyldisilazane (HMDS) prime, prior to forming a coating such as anARC or a photoresist. Such treatment promotes adhesion by changing thesurface tension, and can also affect wettability of the coating bychanging the spreading coefficient. However, even when a coating has apositive spreading coefficient, pinholes can still form when instabilityis present due to long range van der Waals forces.

SUMMARY OF THE INVENTION

According to an aspect of the invention, a film stack is provided inwhich a first film contacts a surface of a substrate at which a givenmaterial is exposed and a second film has an inner surface disposed onthe first film. The second film can have a thickness smaller than areference thickness at which the second film would begin to dewet fromthe surface if the second film directly contacted the surface. Thesecond film can be substantially free of dewetting defects when inliquid phase because it is disposed overlying the first film which has afirst Hamaker constant having a negative value with respect to thesubstrate.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph illustrating free energy and a second derivativethereof for an anti-reflective coating (ARC) as a function of thicknessof the film on a substrate.

FIG. 2 is a diagram illustrating a film stack including an intermediatefilm disposed on a substrate and a top film disposed over theintermediate film.

FIG. 3 is a graph illustrating free energy and a second derivativethereof for an anti-reflective coating (ARC) film as a function ofthickness of the film on a substrate, for cases with, and without anintermediate stabilizing film.

FIG. 4 is a graph illustrating free energy as a function of filmthickness for different materials having positive and negative Hamakerconstants, respectively.

FIG. 5 is a diagram illustrating a determination of free energy for afilm stack having an intermediate film and an overlayer film.

FIG. 6 is a graph illustrating free energy and a second derivativethereof for an anti-reflective coating (ARC) film as a function ofthickness of the film for cases with a small thickness intermediate filmand without.

FIG. 7 is a graph illustrating free energy and a second derivativethereof for an anti-reflective coating (ARC) film as a function ofthickness of the film for cases with an intermediate film having a lowervalue Hamaker constant and without.

FIG. 8 is a graph illustrating free energy for a film stack having anoverlayer of polystyrene for different thicknesses of intermediatefilms.

FIG. 9 is a graph illustrating a second derivative of free energy for afilm stack having an overlayer of polystyrene for different thicknessesof intermediate films.

DETAILED DESCRIPTION

An embodiment of a film stack according to an embodiment of theinvention is illustrated in FIG. 2. According to such embodiment, a filmstack 110 includes a first film 102 disposed on a substrate 100, and ananti-reflective coating (ARC), disposed as a second film 104 over thefirst film 102. The ARC is an example of one of many types of filmsimproved by the present invention, such types including thin filmpolymers, atomic layer deposited (ALD) films, metalorganic chemicalvapor deposition (MOCVD) deposited films, chemical vapor deposition(CVD) deposited films, and physical vapor deposition (PVD) depositedfilms. The film stack 110 is disposed within a medium which overlies theouter surface of the second film. The medium may be a fluid medium,i.e., a gaseous or liquid medium, for example, air or water, thatcontacts an outer surface of the film stack. Alternatively, the mediummay be a solid medium which is subject to a phase change to a fluidmedium at a temperature of interest.

At the particular thickness to which it is deposited, the second film(ARC) ordinarily has a tendency to dewet from the substrate, due to longrange attractive force between the substrate and medium. In the case ofthe solid medium, the second film has a tendency to dewet from thesubstrate when at least a part of the solid medium changes to a fluidphase. However, the presence of the first film 102 between the substrate100 and the second film 104 modifies the long-range attractive force,such that the second film becomes a stable film at that thickness, andis no longer prone to nucleation and growth of holes.

FIG. 3 graphically illustrates a curve representing the free energy 10and the second derivative thereof 14 for a first arrangement asdescribed above relative to FIG. 1, in which the second film (e.g., ARC)is disposed directly on a substrate. The free energy of an arrangementincluding any two materials separated by a film of thickness h is givenby the equation

$\begin{matrix}{{\phi_{vdW}(h)} = {- \frac{A}{12\;\pi\; h^{2}}}} & {{Equation}\mspace{14mu}(1)}\end{matrix}$where φ_(vdW) is the free energy due to van der Waals forces, and A isreferred to as the Hamaker constant. FIG. 4 graphs a curve 50representing φ_(vdW) for different thicknesses in which the film has anegative value Hamaker constant A with respect to the substrate, and acurve 52 representing φ_(vdW) for different thicknesses in which thefilm has a positive value Hamaker constant A with respect to thesubstrate. As apparent from FIG. 4, equation (1) shows that for anymaterial having a positive Hamaker constant, any decrease in thethickness of the film decreases the free energy of the film stack. Thereduction in free energy results in the film becoming unstable anddewetting the substrate below a minimum thickness to minimize theoverall free energy of the system. On the other hand, when the Hamakerconstant is negative, it is apparent from curve 50 that decreasing thethickness of the film actually increases the free energy. Thus, the filmstack remains stable.

The second derivative of the free energy according to Equation (1), isdetermined by

$\begin{matrix}{{\phi_{vdW}(h)} = {- {\frac{A}{2\;\pi\; h^{4}}.}}} & {{Equation}\mspace{14mu}(2)}\end{matrix}$

The zero in this curve determines the minimum thickness below which thedeposited film is subject to spinodally dewetting from the substrate. Asgraphed in FIG. 3, this minimum thickness occurs at 85 nm.

Referring again to FIG. 3, the free energy curve 20 and the secondderivative thereof 22 are shown for a film stack including a secondfilm, illustratively ARC, disposed over an intermediate film, which, inturn, contacts the substrate. The intermediate film modifies thelong-range attractive force between the substrate and the medium, whichis, illustratively, air. In the particular example shown, theintermediate film has a thickness of 4 nm. As shown in FIG. 3, thepresence of the 4 nm intermediate film causes the slope of the freeenergy curve 20 in the second film stack to be dramatically differentfrom that of the first film stack. In the second film stack, the secondfilm (ARC) is now stable at all thicknesses, and shows no tendency toform “pinhole” defects due to dewetting.

An arrangement in which a two-layered film stack is disposed between asubstrate and an overlying medium, e.g., air, has free energy, which isdetermined by the equation

$\begin{matrix}{{\phi_{vdW}(h)} = {{- \frac{A_{2}}{12\;\pi\; h^{2}}} + \frac{A_{2} - A_{1}}{12\;{\pi\left( {h + d} \right)}^{2}}}} & {{Equation}\mspace{14mu}(3)}\end{matrix}$where A₁ is the Hamaker constant of the overlying film having an outersurface contacted by the medium, h is the thickness of the overlyingfilm, A₂ is the Hamaker constant of the under layer film which contactsthe substrate, and d is its thickness. Referring to FIG. 5, the Hamakerconstants are determined individually for each component film of thefilm stack, as if such component film were the only film disposedbetween the substrate and the overlying medium. The Hamaker constant A₁is a measure of the van der Waals component force determined for theoverlying film with respect to the substrate, according to the equation

$\begin{matrix}\begin{matrix}{A_{1} = {A_{x} = {{\frac{3{kT}}{4}\left( \frac{ɛ_{1} - ɛ_{x}}{ɛ_{1} + ɛ_{x}} \right)\left( \frac{ɛ_{2} - ɛ_{x}}{ɛ_{2} + ɛ_{x}} \right)} +}}} \\{\frac{3h\;\upsilon_{e}}{8\sqrt{2}}\frac{\left( {n_{1}^{2} - n_{x}^{2}} \right)\left( {n_{2}^{2} - n_{x}^{2}} \right)}{\left( {n_{1}^{2} + n_{x}^{2}} \right)^{\frac{1}{2}}\left( {n_{2}^{2} + n_{x}^{2}} \right)^{\frac{1}{2}}\left\{ {\left( {n_{1}^{2} + n_{x}^{2}} \right)^{\frac{1}{2}} + \left( {n_{2}^{2} + n_{x}^{2}} \right)^{\frac{1}{2}}} \right\}}}\end{matrix} & {{Equation}\mspace{14mu}(4)}\end{matrix}$

where ε₁ is the permittivity of the medium, ε₂ is the permittivity ofthe substrate, and ε_(x) the permittivity of said first film, and n₁,n₂, and n_(x) are the indices of refraction of the medium, thesubstrate, and said first film, respectively, k is the Boltzmannconstant, T is temperature, h is Planck's constant and v_(e) is the mainelectronic absorption frequency (usually 3.0×10¹⁵s⁻¹). The Hamakerconstant A₂ is a measure of the van der Waals component force determinedfor the under layer film with respect to the substrate, according to theequation

$\begin{matrix}{A_{2}\; = {A_{y}\; = \;{{\frac{3\;{kT}}{4}\;\left( \frac{ɛ_{1}\; - \; ɛ_{y}}{ɛ_{1}\; + \; ɛ_{y}} \right)\;\left( \frac{ɛ_{2}\; - \; ɛ_{y}}{ɛ_{2}\; + \; ɛ_{y}} \right)} +}}} \\{\frac{3\; h\;\upsilon_{e}}{8\;\sqrt{\; 2}}\;\frac{\left( {n_{1}^{2}\; - \; n_{y}^{2}} \right)\;\left( {n_{2}^{2}\; - \; n_{y}^{2}} \right)}{\left( {n_{1}^{2}\; + \; n_{y}^{2}} \right)^{\frac{1}{2}}\;\left( {n_{2}^{2}\; + \; n_{y}^{2}} \right)^{\frac{1}{2}}\;\left\{ {\left( {n_{1}^{2}\; + \; n_{y}^{2}} \right)^{\frac{1}{2}}\; + \;\left( {n_{2}^{2}\; + \; n_{y}^{2}} \right)^{\frac{1}{2}}} \right\}}}\end{matrix}$where ε₁ is the permittivity of the medium, ε₂ the permittivity of thesubstrate, and ε_(y) the permittivity of said second thin film, and n₁,n₂, and ny are the indices of refraction of the medium, the substrate,and said second film, respectively. In the above equations, thepermittivities of the substrate, film and medium (air) can be replacedby the dielectric constants k_(i)=ε_(i)/ε₀ for each, since thepermittivities are appear only in unitless factors.

The amount of stabilization achieved for a given film stack according tothe method described herein is related to the thickness and dielectricproperties of the intermediate film. The result of reducing thethickness of the intermediate film from 4 nm to 2 nm is illustrated inFIG. 6. In such case, the film stack including the 2 nm intermediatefilm is not stable for all thicknesses of the second film (ARC).However, the free energy curve 30 and the second derivative 34 thereofare shifted to the left of their original positions 10 and 14, and nowshow stability at a smaller thickness than before. The presence of thezero 32 in the second derivative curve 34 for the second film stack nowindicates the second film (ARC) to have a minimum thickness of 60 nm toavoid spinodal dewetting, as compared to the original minimum thicknessof 85 nm for the film stack having no intermediate film.

FIG. 7 illustrates another example of a film stack in which thethickness of the intermediate film is maintained constant at 4 nm, butin which the dielectric properties of the intermediate film do not asstrongly affect the long-range attractive force between the substrateand the air. Accordingly, the free energy curve 40 and the secondderivative 44 thereof are shifted to the left by about the amount asthey are for the film stack having the 2 nm intermediate film, asgraphed above in FIG. 6. In this case, the film stack is subject tospinodal dewetting at a thickness below 59 nm, as determined by thelocation of the zero 42 in the second derivative curve 44.

An exemplary film stack according to an embodiment of the invention willnow be described, with respect to FIGS. 8 and 9. FIG. 8 is a graphdepicting free energy curves for a film stack in which a top film ofpolystyrene is disposed on an intermediate film ofpolytetraflouroethylene (PTFE), which in turn, is disposed on asubstrate of silicon dioxide. The properties of these materials are:

n_(SiOx) = 1.46 n_(Polystyrene) = 1.557 n_(PTFE) = 1.359 ε_(SiOx)/ε₀ =3.9 ε_(Polystyrene)/ε₀ = 2.55 ε_(PTFE)/ε₀ = 2.1

From these constants, the Hamaker constant is calculated for each of thepolystyrene and PTFE materials individually, with respect to anarrangement including the substrate, only one of the polystyrene andPTFE materials and the overlying medium (air) which contacts the outersurface of the film stack. The results of these calculations are:A _(Polystyrene)=1.358×10⁻²⁰ J, and A _(PTFE)=−1.070×10⁻²⁰ J.

Because of the positive value of its Hamaker constant, it is clear thatpolystyrene would be unstable for small thicknesses, if disposeddirectly on the silicon dioxide substrate. On the other hand, thenegative value of the Hamaker constant for the PTFE film indicates thatit would be stable at all thicknesses. Equation (3) above can be used todemonstrate that an intermediate film of PTFE disposed between apolystyrene film and a silicon dioxide substrate stabilizes thepolystyrene film. A graph illustrating the free energy of the filmstack, for different thicknesses of the PTFE intermediate film, isprovided in FIG. 8. As shown therein, when the film stack lacks the PTFEintermediate film, the free energy curve 80 is negative for allthicknesses. Free energy is negative for small thicknesses of thepolystyrene film when the PTFE film has thickness of 5 nm (curve 82). Onthe other hand, curves 84, 86, 88, and 90 indicate that the free energyremains positive for film stacks having polystyrene films of smallthickness, when the PTFE film has thicknesses of 10 nm, 15 nm, 20 nm,and 25 nm, respectively.

However, determining the thicknesses of the polystyrene and PTFE filmswhich correspond to a stable film stack can be difficult by determiningthe free energy according to Equation (3). The film thicknesses at whichthe film stack transitions between stability and instability are morereadily determined from the following equation which is the secondderivative of Equation (3)

$\begin{matrix}{{\phi_{vdW}^{''}(h)} = {{- \frac{A_{2}}{2\pi\; h^{4}}} + \frac{A_{2} - A_{1}}{2{\pi\left( {h + d} \right)}^{4}}}} & {{Equation}\mspace{14mu}(5)}\end{matrix}$

As indicated in the foregoing, when φ_(vdW)″(h), the second derivativeof the free energy equation, is positive, then the film stack is stable.However, when φ_(vdW)″(h) is negative, the film stack is unstable. Thesetransitions are apparent from an examination of FIG. 9 and Table 1.

TABLE 1 Polystyrene Film Stable Below PTFE Thickness (nm) Thickness (nm)0 All Unstable 5 20 10 42 15 64 20 86 25 >100 nm

As indicated by curve 200 in FIG. 9 and the first entry of Table 1, whenthere is no PTFE film, the film stack is unstable for all thicknesses ofthe polystyrene film. However, as indicated by curve 202 and the secondentry of Table 1, when the PTFE film has a thickness of 5 nm, thepolystyrene film is stable for only for a thickness of less than 20 nm.As the thickness of the PTFE is increased to 10 nm (curve 204) and more,the polystyrene film becomes stable for thicknesses of the polystyrenefilm greater than 20 nm. For example, when the thickness of the PTFEfilm is 15 nm thick (curve 206), the polystyrene film is stable for allthicknesses less than 64 nm. When the PTFE film is 20 nm thick (curve208), the polystyrene film is stable at an even greater thickness. At aPTFE film thickness greater than 25 nm (curve 210), the polystyrene filmis stable for thicknesses from zero to beyond 100 nm.

The exemplary film stack including polystyrene and PTFE, as described inthe foregoing, is only one example. According to the principles ofembodiments of the invention described herein, any film that is subjectto dewetting from a substrate can be stabilized by the addition of anappropriate stabilizing film. The Optical and dielectric properties ofthe stabilizing film, as well as its thickness, are factors indetermining the value and magnitude of the stabilization achieved by thefilm, as represented by the Hamaker constant.

While the invention has been described in accordance with certainpreferred embodiments thereof, those skilled in the art will understandthe many modifications and enhancements which can be made theretowithout departing from the true scope and spirit of the invention, whichis limited only by the claims appended below.

1. A film stack, comprising: a substrate having a surface and a givenmaterial exposed at said surface; a first thin film including a firstpolymer contacting said given material at said surface of saidsubstrate; and a second thin film including a second polymer other thansaid first polymer, said second thin film having an inner surfacecontacting said first thin film, said second thin film beingsubstantially free of dewetting defects when in liquid phase and saidsecond thin film consisting essentially of a material having adielectric constant k higher than about four times the dielectricconstant of free space, wherein said second thin film has a subminimumthickness being smaller than a reference thickness at which said secondthin film would begin to dewet when in liquid phase if said second thinfilm directly contacted said surface due to long range van der Waalsattractive force between said given material and a medium to which saidsecond thin film is exposed and said first thin film has a negativevalue Hamaker constant relative to said substrate so as reduce said longrange van der Waals attractive force, thereby avoiding said subminimumthickness second thin film from beginning to dewet.
 2. A film stack asclaimed in claim 1, wherein said reference thickness at which saidsecond thin film would begin to dewet is defined as the thickness atwhich the quantity${\phi_{vdW}^{''}(h)} = {{- \frac{A_{2}}{2\pi\; h^{4}}} + \frac{A_{2} - A_{1}}{2{\pi\left( {h + d} \right)}^{4}}}$is zero, if said second thin film directly contacted said surface, whereφ_(vdW)″(h) is the second derivative of free energy of the system, A₁ isthe Hamaker constant of said first thin film with respect to said givenmaterial, A₂ is a Hamaker constant of said second thin film with respectto the substrate, h is a thickness of said first film, and d is athickness of said second thin film.
 3. A film stack as claimed in claim2, wherein said Hamaker constant A₁ of said first thin film is afunction of the indices of refraction and permittivities of said firstthin film and the substrate and said Hamaker constant A₂ of said secondthin film is a function of the indices of refraction and permittivitiesof said second thin film and said given material.
 4. A film stack asclaimed in claim 3, wherein said value of said Hamaker constant A₁ isdetermined according to the equation $\begin{matrix}{A_{1} = {A_{x} = {{\frac{3{kT}}{4}\left( \frac{ɛ_{1} - ɛ_{x}}{ɛ_{1} + ɛ_{x}} \right)\left( \frac{ɛ_{2} - ɛ_{x}}{ɛ_{2} + ɛ_{x}} \right)} +}}} \\{\frac{3h\;\upsilon_{e}}{8\sqrt{2}}\frac{\left( {n_{1}^{2} - n_{x}^{2}} \right)\left( {n_{2}^{2} - n_{x}^{2}} \right)}{\left( {n_{1}^{2} + n_{x}^{2}} \right)^{\frac{1}{2}}\left( {n_{2}^{2} + n_{x}^{2}} \right)^{\frac{1}{2}}\left\{ {\left( {n_{1}^{2} + n_{x}^{2}} \right)^{\frac{1}{2}} + \left( {n_{2}^{2} + n_{x}^{2}} \right)^{\frac{1}{2}}} \right\}}}\end{matrix}$ where ε₁ is the permittivity of the medium, ε₂ thepermittivity of the substrate, and ε_(x) the permittivity of said firstthin film, n₁, n₂, and n_(x) are the indices of refraction of themedium, the substrate, and said first thin film, respectively, and saidvalue of said Hamaker constant A₂ is determined according to theequation $\begin{matrix}{A_{2}\; = {A_{y}\; = \;{{\frac{3\;{kT}}{4}\;\left( \frac{ɛ_{1}\; - \; ɛ_{y}}{ɛ_{1}\; + \; ɛ_{y}} \right)\;\left( \frac{ɛ_{2}\; - \; ɛ_{y}}{ɛ_{2}\; + \; ɛ_{y}} \right)} +}}} \\{\frac{3\; h\;\upsilon_{e}}{8\;\sqrt{\; 2}}\;\frac{\left( {n_{1}^{2}\; - \; n_{y}^{2}} \right)\;\left( {n_{2}^{2}\; - \; n_{y}^{2}} \right)}{\left( {n_{1}^{2}\; + \; n_{y}^{2}} \right)^{\frac{1}{2}}\;\left( {n_{2}^{2}\; + \; n_{y}^{2}} \right)^{\frac{1}{2}}\;\left\{ {\left( {n_{1}^{2}\; + \; n_{y}^{2}} \right)^{\frac{1}{2}}\; + \;\left( {n_{2}^{2}\; + \; n_{y}^{2}} \right)^{\frac{1}{2}}} \right\}}}\end{matrix}$ where ε₁ is the permittivity of the medium, ε₂ thepermittivity of said given material, and ε_(y), the permittivity of saidsecond thin film and n₁, n₂, and n_(y), are the indices of refraction ofthe medium, said given material, and said second thin film respectively.5. A film stack as claimed in claim 4, wherein said second thin filmincludes a bottom anti-reflective coating (BARC) material.
 6. A filmstack as claimed in claim 1, wherein said first thin film has athickness where said first thin film is disposed between said substrateand said second thin film and said thickness of said second thin film isgreater than said Thickness of said first thin film.
 7. A film stack asclaimed in claim 1, wherein said thickness of said second thin film isless than 200 nanometers (nm).